7.9 Cap on Nov 17

On Nov 17 a series of quake struck the Aleutian Islands. Of the dozens of quakes, the cap seemed to be 7.9. Quakes of 8.0 or higher put the affair
in a different, to be avoided, category of earthquake statistics. How did it happen that 4 quakes, among the dozens, were 7.9 but not one dared
sneak higher.